The sensor is a fluorescence sensor, for example. The problem upon which the application for patent is based will now be discussed in terms of a fluorescence measurement. In order to measure fluorescence one generally irradiates the medium with a short-wavelength excitation light and detects the longer-wavelength fluorescent light produced by the medium. A fluorescence measurement is typically arranged as depicted in FIG. 1.
For this purpose, the fluorescence sensor 3 comprises a light source 1 and a receiver 2. The light source 1 transmits transmission light, while the receiver 2 receives reception light. Since the fluorescence light radiates in all directions, the light paths of the transmission light and the reception light may, in principle, stand at any angle α with respect to each another. An angle of 0° is not suitable, since, in this instance, the weak fluorescence light is superimposed by the strong excitation light. 90° is quite suitable, provided there is sufficient space to arrange the components. If the measurement array is to be installed in a probe tube, an angle as close as possible to 180° would be chosen, so that light source 1 and receiver 2 are arranged closer together. An angle of 180° is manageable with the help of a beam splitter 4, as can be seen in FIG. 2. For instance, this principle is embodied in the product, “FP360 sc PAK/Öl Fluoreszenz-Sonde [PAK/Oil Fluorescence Probe],” from Hach. However, in this instance, the angle of 180° is present only between the beam splitter 4 and the medium 5. The receiver 2 is again arranged perpendicular to the direction of irradiation and therefore requires lateral installation space, which is often not available.
This problem, however, generally occurs with other optical measurements as well, for instance, with scattered light or absorption measurements.